Accommodate different chipmunk install location.

Removed cgo CFLAGS directive with hard-coded include path.  Added
"chipmunk/" to C includes to allow CGO_CFLAGS to point to a different
general location, e.g. "-I/usr/local/include".
body: Fixed EachArbiter callback parameter type.
Merge pull request #1 from ianremmler/fixes

Fixes